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Adding integer data with single pointer

Posted on 2006-11-09
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Last Modified: 2010-04-15
If you have an array of integers, is it legal (or recommended) to get a sum by iterating through the array with a pointer, and adding the value pointed to by the next pointer, to the value pointed to by the current pointer?  Like this:

      int* ptr = (int*) malloc(3 * sizeof(int));

      ptr[0] = 66;
      ptr[1] = 50;
      ptr[2] = 1000;

      int* sum = ptr;
      for (unsigned i = 0; i < 2; ++i)
            *sum += *(++sum);

      printf("%d\n",*sum);

This works, but I wonder if it's ANSI compliant, or if it is even a good idea.  What sort of assembly code would this generate?  Would it be more efficient than using a temporary value and incrementing that?
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Question by:chsalvia
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Expert Comment

by:sunnycoder
ID: 17911892
Hi chsalvia,

No it would not work ...
*sum += *(++sum);
This modifies the value *sum more than once between two sequence points ...
http://publications.gbdirect.co.uk/c_book/chapter8/sequence_points.html

Cheers!
sunnycoder
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sunnycoder earned 1000 total points
ID: 17911913
oops .. I posted too soon ... ++sum modifies the pointer and += modifies the *sum, so this should be fine ... note that you are chaging the values in your array to get cumulative sum
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Author Comment

by:chsalvia
ID: 17911922
I see.  So you'd need to do:

      int sum = 0;
      for (unsigned i = 0; i < 3; ++i)
            sum += *(ptr++);
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LVL 45

Expert Comment

by:sunnycoder
ID: 17911929
yes, that would not modify the contents of the array and yet give you the sum.
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Expert Comment

by:ozo
ID: 17912065
*sum += *(++sum); modifies sum only once, but it violates the requirement that the prior value shall be read only to determine the value to be stored, so it's still illegal
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